application/xhtml+xml was added to the Accept header in order to enable the serving of MathML to both Mozilla and IE with Apache without scripting back when the MathPlayer plug-in for IE did not handle application/xhtml+xml.

Now the real purpose of (X)HTML in the next quote.

If your document mixes MathML or SVG with XHTML, you should use application/xhtml+xml (until HTML5 parsing is supported).

However, if you are using the usual HTML features (no MathML or SVG) and are serving your content as text/html to other browsers, there is no need to serve application/xhtml+xml to Mozilla. Serving valid HTML as text/html ensures the widest browser and search engine support.

Both of these samples below works fine on my Mozilla Firefox 3.6.6+ running Arch Linux machine. Except this unicode character, &InvisibleTimes; (fileformat.info). Similar to &#8290; in decimal and &#x2062; in hex.

Below are samples of MathML markups from Henri Sivonen post on that hacks.mozilla.org–adapted using decimal entity:

x=−b&PlusMinus;b2−4⁢a⁢c2⁢a

Below is SVG sample. The right padding looks bad on Presto (Opera). Sample of SVG (text-based image in left float) from the same Henri’s post above:

The markups slightly modified for this blog doctype.

Promising. But HTML5 brings the euforia for the Apple’s team also. Read this Christopher Blizzard–Firefox evangelist–post titled Intellectual Honesty and HTML5 on his weblog (0xdeadbeef.com):

That’s right. If you’re not on Safari, then F*ck You.

Aside from the incendiary language I’ve used to help you understand how it feels the real underlying message here is that if you don’t have access to Safari then you must not have access to HTML5. Wait, only Safari supports HTML5??

Some vendor specific features are still in draft for the open Web standards.

HTML5 is in a dangerous place since everyone wants to own it, but everyone is in a different place in terms of support or even what it means. I can’t promise what other organizations will do, but I can at least say what I will do in the future. At Mozilla, intellectual honesty matters and it matters to me personally. So I don’t think you’ll see us do things like this in the future. To us, the web and its users matter more than any particular standard or browser. And you’ll see that reflected in messaging that comes from me and shows up as marketing.